With generic rack and pinion steering gears, a steering shaft transfers the steering torque applied to a steering wheel by a driver to an input shaft of a steering gear. The input shaft of the steering gear is designed as a pinion and meshes with a gear rack which is movably mounted in the longitudinal direction in the housing of the steering gear. A rotation of the input shaft and of the pinion is thereby converted into a sliding movement of the gear rack. The gear rack itself is connected to the steered wheels of the motor vehicle via track rods. The turning of the steering wheel is in this way converted into a steering movement of the steered wheels.
The meshing of the gear teeth between the pinion and the gear rack should be as free from play as possible. In order to achieve this, the gear rack is usually pressed against the pinion by a spring-loaded pressure piece. The pressure piece itself abuts on the surface of the gear rack facing away from the gear teeth and sits in a generally cylindrical bore of the steering gear housing. The simplest form of the pressure piece is a metallic pressure piece which is plain bearing mounted in the cylindrical bore and pretensioned against the gear rack by a coil spring. Due to the friction between the gear rack and the pressure piece, when the gear rack is moved back and forth such a pressure piece is alternately pressed against the walls of the bore in the movement direction of the gear rack. In the process, noises-so-called pressure piece rattling-occur.
Various approaches are known for reducing or completely preventing pressure piece rattling. With some solutions, a plastic or rubber element is inserted between the pressure piece and the pressure piece bore, which damps the movement of the pressure piece and is to prevent direct metal contact. A rack and pinion steering gear is therefore known from the patent specification EP 1 084 933 B1, in which it is ensured that a pressure piece is guided free from rattling in a bore of a steering gear housing via an o-ring arranged in an eccentric groove. The publication DE 2 634 054 A1 describes a pressure piece which is mounted itself in a metallic casing and is pressed against the gear rack by a coil spring acting between the casing and the pressure piece. The metallic casing is in turn provided with a rubber bushing on its outer circumferential surface, which supports the casing in the radial direction of the bore against the steering gear housing. Direct metal contact between the casing and the housing is to be prevented in this way. The rubber casing is not or only slightly stressed in the axial direction of the bore, since the metallic casing does not follow the axial movements of the pressure piece. This design is complex and complicated to assemble. A pressure piece is known from the publication DE 10 2009 028 031 A1 which has damping elements consisting of an elastomer arranged on its edge. These damping elements prevent direct metal contact between the pressure piece and the steering gear housing. Assembling this pressure piece, in which the elastomers are mounted under pretension, is complicated. In addition, there is the risk that through movements of the pressure piece wearing of the elastomers occurs.
The publication DE 10 2004 004 390 A1 shows another way. Here, a pressure piece is proposed which holds a metallic tolerance strip in a circumferential groove, by means of which the pressure piece can be inserted fitting perfectly and free from play into the bore. The free from play assembly and the purely metallic components are not supposed to damp the relative movement of the pressure piece with respect to the housing, which leads to rattling noises, but fully prevent it. This technical solution is very complicated to manufacture, since after the manufacture of the pressure piece and the steering gear housing the tolerances have to be measured and then a correspondingly fitting tolerance strip has to be selected, by means of which the fitting is adjusted free from play but not sticking. In addition to the precise measurement of the components during manufacture, a large number of tolerance strips of different dimensions are required to be held available and selected from.
DE 10 2006 023 795 A1 shows a pressure piece arrangement having an outer casing which can be pressed in a receiving bore of the steering gear by means of a press fit. A pressure piece is arranged in the casing and is pressed against the gear rack by means of an annular elastomer spring and a coil spring. The elastomer spring only acts in the axial direction of the arrangement. It does not surround or enclose the pressure piece. The pressure piece is instead guided directly in the case.
DE 102 47 331 A1 shows a pressure piece unit consisting of a casing which is pressed into a receiving bore of a steering gear. The pressure piece is supported laterally with respect to the casing via an o-ring inserted loose into a groove. Relative movements occur between the o-ring and the casing in operation which cause friction and wear. The o-ring does not produce any axial contact pressing force. The contact pressing force of the pressure piece on the gear rack is produced via a coil spring. The pressure piece has a particularly low noise level owing to its manufacture from a plastic material.
DE 2 049 386 describes a pressure piece which is pressed against the gear rack via an elastomer spring. Here, the elastomer spring only replaces the coil spring which is usually provided. The elastomer spring has an outer casing, a tubular elastomer and an inner casing. The spring action occurs by shearing stress, or more precisely by shearing forces, when the two metallic casings are displaced against one another in the axial direction. In this document, the pressure piece itself is directly guided in a receiving bore of the steering gear housing and is in direct metal contact with it. In operation, a relative movement takes place between the pressure piece and the receiving bore in the housing. This causes friction and wear.
The known solutions are either complex during production and assembly or they are prone to pressure piece rattling.
Therefore, it is the object of the present invention to create a rack and pinion steering gear, in which the pressure piece rattling does not occur and which is nevertheless easy to assemble and has low wear in operation.